Abstract: A hydraulic drive system and a bicycle having such system as its drive train, the system including a pump, preferably a vane pump, operably connected to the bicycle pedal crank, the pump delivering pressurized fluid to a hydraulic motor operably connected to the wheel or mechanism to be driven, e.g., the bicycle drive wheel. The pump has an impeller eccentrically disposed in a housing, and a piston-cylinder mechanism is provided to change the eccentricity of the pump impeller with respect to the pump housing as a function of hydraulic system pressure, to thereby maintain a constant input force, e.g., pedal pressure, and rotation rate, for a wide range of speeds. The motor also has an impeller eccentrically disposed in a housing, and a spacer bar connected between the pump and motor housings automatically adjusts the motor impeller eccentricity with changes in the pump impeller eccentricity, thereby further maintaining constant pedal pressure and rotation rate for a wide range of speeds.